text: Proteins make up the remaining 10% of the surfactant.  Half of this 10% is plasma proteins but the rest is formed by the apolipoproteins, surfactant proteins SP-A, SP-B, SP-C, and SP-D. The apolipoproteins are produced by the secretory pathway in type II cells. They undergo much post-translational modification, ending up in the lamellar bodies. These are concentric rings of lipid and protein, about 1 µm in diameter.
  * SP-A and SP-D are collectins. They confer innate immunity as they have carbohydrate recognition domains that allow them to coat bacteria and viruses promoting phagocytosis by macrophages. SP-A is also thought to be involved in a negative feedback mechanism to control the production of surfactant.
  * SP-B and SP-C are hydrophobic membrane proteins that increase the rate that surfactant spreads over the surface.  SP-B and SP-C are required for the proper biophysical function of the lung. Humans and animals born with a congenital absence of the saposin family SP-B experience intractable respiratory failure whereas those born lacking SP-C tend to develop progressive interstitial pneumonitis.
  The SP proteins reduce the critical temperature of DPPC's phase transition to a value lower than 37 °C, which improves its adsorption and interface spreading velocity. The compression of the interface causes a phase change of the surfactant molecules to liquid-gel or even gel-solid. The fast adsorption velocity is necessary to maintain the integrity of the gas exchange region of the lungs.
  Each SP protein has distinct functions, which act synergistically to keep an interface rich in DPPC during lungs expansion and contraction. Changes in the surfactant mixture composition alter the pressure and temperature conditions for phase changes and the phospholipids crystal shape as well. Only the liquid phase can freely spread on the surface to form a monolayer. Nevertheless, it has been observed that if a lung region is abruptly expanded the floating crystals crack like "icebergs". Then the SP proteins selectively attract more DPPC to the interface than other phospholipids or cholesterol, whose surfactant properties are worse than DPPC's. The SP also fastens the DPPC on the interface to prevent the DPPC from being squeezed out when the surface area decreases  This also reduces the interface compressibility.